Plastics are widely used in late years as materials for automobile parts, electric parts and building materials because they have lots of advantages such as light weights, rust resistance and wide flexibility of design. In particular, polyolefin-based resins are widely used as industrial materials because they have many excellent properties such as low cost, formability, chemical resistance, heat resistance, water resistance, and electric characteristics, and their demand growth in future market is also expected. However, polyolefin-based resins are non-polar and crystalline, so that difficulty in adhesion also arises in contrast to polar synthetic resins.
To address these disadvantages, as a pre-treatment of coating and adhesion, the surface of configured articles is activated by plasma treatment or gas flame treatment. Alternatively, a primer (adhesion promoter) whose main component is chlorinated polyolefin is applied.
As a primer for coating polypropylene bumpers of automobiles, for example, primer compositions whose main component is chlorinated modified polyolefin are disclosed (Patent Documents 1 and 2). These primers formed of chlorinated resins are excellent in adhesion strength to conventional polyolefin. However, some of the substrates which have recently appeared do not sufficiently adhere because of high rigidity of the substrates and low temperature baking performed thereon. Thus, in the state of art, the conventional chlorinated polyolefin resins can not sufficiently deal with the new substrates. In addition, the chlorinated polyolefin resins exhibit relatively good adhesion strength to the polypropylene substrates, but have insufficient adhesion strength to other substrates (polyvinyl chloride, polycarbonate, PET, ABS, nylon).
The chlorinated polyolefin resins proposed hitherto have as the main component a chlorinated isotactic polypropylene (IPP) produced by a Ziegler-Natta catalysis polymerization, and then chlorinating IPP. On the contrary, there are disclosed adhesives using chlorinated syndiotactic polypropylene (SPP) produced by a metallocene catalysis polymerization, and then chlorinating SPP (Patent Documents 3 and 4). Although this chlorinated SPP are superior in solubility in solvent compared with the chlorinated IPP produced by Ziegler-Natta catalysis polymerization, the chlorinated SPP has a weakness that it exhibits such an excellent adhesion strength to only the polypropylene substrates and insufficient adhesion strength to the other substrates (e.g., polyvinyl chloride, polycarbonate, PET, ABS, nylon).
In order to address these problems, development is being made aiming at exhibiting excellent adhesion strength under a condition of the low temperature baking at 80° C. (Patent Document 5). However, further lowering the baking temperature is demanded in recent years. It has been recently revealed that sufficient results are not always obtained when the baking temperature is lowered to 60° C. or below.
Although the chlorinated resin has conventionally been used in a form of a solution in an aromatic organic solvent such as toluene and xylene, recently there have been made an attempt to use aqueous solution in order to address environmental problems and safe hygiene (Patent Documents 6, 7, 8 and 9). However, these waterborne resins require numerous energy and time for drying and baking steps after applying the resins onto the substrates, when compared with organic solventborne resins. To solve this problem, there is an increasing demand for a high-solid chlorinated resin waterborne dispersion which can be used in low temperature baking.
Polyolefin substrates tend to be having high rigidity recently. The waterborne dispersion of conventional chlorinated resin can not exhibit sufficient adhesion strength to such a substrate, and thus it is becoming difficult to ensure sufficient adhesion with such a substrate. Furthermore, for application on the automobile parts, there is also a demand for gasohol resistance, and it is therefore more difficult to ensure applicability.
As one of means to deal with the low temperature baking, lowering of a softening temperature of a raw polypropylene is effective. However, in the conventional polymerization using the Ziegler-Natta catalyst, it is necessary to increase a composition ratio of ethylene or other α-olefins in order to lower the softening temperature. As a result, the properties such as adhesion strength and gasohol resistance are decreased. If a molecular weight of the chlorinated resin is increased in order to compensate the decrease of properties, it is not well-emulsified or dispersed due to the increased melt viscosity during production of the waterborne composition. Due to the high viscosity of the final product, such a strategy is not suitable for obtaining a high-solid product.
Conventional copolymers of polypropylene or propylene with ethylene or other α olefins have a broad molecular weight distribution, and a broad molecular weight distribution after acid modification and chlorination as well. Existence of relatively low molecular weight components reduces adhesion strength and solvent resistance. Particularly, the gasohol resistance is remarkably deteriorated. In order to improve this defect, it is necessary to remove low molecular weight components by extracting the same in solvents, which is uneconomical.
Meanwhile, syndiotactic polypropylene (SPP) produced with a metallocene catalyst is characterized by having low softening temperature and narrow molecular weight distribution, and the technology of the primer composed of the chlorinated resin using this SPP is disclosed (Patent Document 3). However, sufficient adhesive strength is not obtained because most of substrates contain as a main component isotactic polypropylene (IPP) that is produced with the Ziegler-Natta catalyst.
As explained above, with the waterborne dispersion composed of the conventional chlorinated resin, it was not possible to obtain good adhesive strength and gasohol resistance with ensuring applicability to high solidification and low temperature baking.
In order to address such problems in the waterborne dispersion, the technology is being developed to achieve excellent adhesion strength in the low temperature baking condition at 80 to 90° C. (e.g., patent Document 10). However, in recent years, there is a further demand for lowering of the baking temperature of the waterborne dispersion. It has been recently revealed that satisfied results are not always obtained when the baking temperature is lowered to, e.g., 60° C. or below.    Patent Document 1: JP-S57-36128 A    Patent Document 2: JP-S63-36624 B    Patent Document 3: JP-3045498 B    Patent Document 4: JP-H7-18016 A    Patent Document 5: JP-2003-321588 A    Patent Document 6: JP-H8-6009 B    Patent Document 7: JP-H5-209006 A    Patent Document 8: JP-2769958 B    Patent Document 9: WO90/12656 Pamphlet    Patent Document 10: JP-2003-327761 A    Patent Document 11: JP-2001-206914 A